1. Field of the Invention:
The present invention relates to a process for producing vinylchloride monomer (hereinafter designated as VCM) by the pyrolysis of 1,2-dichloroethane (hereinafter designated as EDC) under pressure.
2. Discussion of Background:
Prior art methods for the production of VCM teach that purified EDC in a liquid form is introduced in a preheated zone of the pyrolysis furnace under a pressure and, after being evaporated in an evaporation zone, is thermally decomposed at a temperature range from 480 to 550.degree. C. in a pyrolysis reaction zone, to produce VCM.
In this treatment the cracked gas at a high temperature which flows out of the pyrolysis furnace mostly contains VCM and hydrogen chloride produced by the pyrolysis and the EDC remaining undecomposed. Usually the abundant heat of the cracked gas at a high temperature is exhausted away from the system with cooling water in a quencher which follows in the operation line. In the next step of operation, hydrogen chloride and the EDC remaining undecomposed are separated from VCM by use of a distillation column.
When the high temperature furnace for pyrolysis of EDC is operated for a long period of time, rigid coke is generally formed on the inside of the pyrolysis tube line. Therefore it is necessary to stop the operation before the pressure loss of the pyrolysis furnace itself exceeds the tolerance limit and decoke.
The rate of pyrolysis of EDC, or the yield of VCM per unit amount of EDC introduced in the pyrolysis furnace, can be enhanced by elevating the temperature at which the pyrolysis reaction takes place. As a result of the treatment, however, by-products such as methylchloride and butadiene which may cause trouble by polymerization are also increased. These by-products require a large amount of energy to be removed in the following stage.
Further, in the pyrolysis furnace where EDC is preheated, evaporated and decomposed by heat, the elevated temperature inside the furnace promotes the formation of rigid coke on the inside wall of the tubes, which consequently shortens the life of the pyrolysis furnace.
In addition, the elevated temperature at which the exhaust gas after combustion is exhausted from the pyrolysis furnace brings about a large increase in the loss of thermal energy.
For these reasons, temperature of the pyrolysis reaction of EDC in the actual operations is limited by the amounts of by-products and the coke which may be formed. In other words, the rate of pyrolysis can not reasonably be increased beyond a certain level.
For reducing the amount of coking formed in the tubes of the pyrolysis furnace, Japanese Laid-Open patent application No. Sho 49-125306 proposes a process in which EDC is evaporated on the outside of the pyrolysis furnace or, in other words, gaseous EDC is introduced into the pyrolysis furnace.
This process, however, requires a high temperature heat source for the evaporation of EDC and therefore a heating furnace for EDC is needed in addition to the pyrolysis furnace.
Further, additional problems of the heating furnace itself are deterioration of EDC due to the high temperature and necessity for removing scales. Thus, the expense for the operation and equipment result in an increase in the cost.
On the other hand, the cracked gas flowing out of the pyrolysis furnace contains a tremendous amount of heat energy, so that tremendous heat energy is lost to the cooling water in a quencher located immediately after the pyrolysis furnace.
A variety of methods have been proposed for recovering and utilizing the energy of the high temperature cracked gas flowing out of the pyrolysis furnace.
For instance, Japanese Laid-Open patent application No. Sho 56-45424 describes a method in which a cracked gas flowing out of a pyrolysis furnace is cooled in at least two steps and the heat transferred to a heat transfer medium is used to heat other devices.
The heat transfer medium should be selected based on consideration of resistance against the high temperature deterioration and the specific properties of the gas flowing out of the pyrolysis furnace. Precautions must be taken to avoid explosion on leakage due to hydrogen chloride or other compounds contained in the gas.
In particular, when water or stream is used as heat transfer medium, leakage of hydrogen chloride may cause severe corrosion and even breakage of equipment. In this case, the apparatus should be reconstructed on a larger scale, to utilize the recovered heat.
Japanese Laid-Open patent application No. Sho 55-129233 discloses a process in which the heat of cracked gas is indirectly exchanged using a heat transfer medium and the heat transfer medium is employed as a heat source for a distillation column for EDC, as an example of using the heat transfer medium for a heat source in another operation.
Here, troublesome operations are usually required, for example, because the distillation column is under the influence of the variations in the operating conditions of the pyrolysis furnace. Furthermore, since a pyrolysis furnace usually is situated some distance apart from a distillation column, this operation necessarily requires a large expense for equipment.
Another process is also proposed in which the exchange of heat between a cracked gas flowing from a pyrolysis furnace and EDC supplied to the pyrolysis furnace is utilized to preheat or evaporate the EDC.
In this case, however, the cracked gas flows out at a relatively high rate and therefore a fairly large length is necessary for the heat transmission tube of the heat exchanger. Consequently a pressure drop in the tubes results with the heat exchanger which is located immediately after the pyrolysis furnace and this shortens the time for a continuous operation of the pyrolysis furnace. Thus, to overcome the difficulties, a large scale remodeling of the pyrolysis furnace is necessary, for example, to expand the diameter or shorten the length of the heat transmission tubes.
In order to raise the rate of pyrolysis without elevating the temperature of the pyrolysis reaction, or in other words without increasing the amount of by-products formed in the pyrolysis furnace and a loss in thermal energy, it is necessary to increase the area of reaction zone in the pyrolysis furnace without making any drastic remodeling of the furnace. Also for the reduction of coking in tubes of the pyrolysis furnace, gaseous EDC should be introduced in the pyrolysis furnace at a site which lies in a range of zones called (for liquid EDC) from the preheating zone to the evaporation zone, preferably at the entrance of the preheating zone.
The cracked gas flowing out of the pyrolysis furnace contains large amounts of thermal energy which can be recovered by exchanging heat between the cracked gas and supplied EDC and be utilized for preheating and evaporating the EDC.
However, the heat exchange operation requires an effective heat exchanger which is installed immediately after the pyrolysis furnace and performs the exchange of heat between the cracked gas and supplied EDC with an operational condition set so as to minimize the pressure drop.
For prolonged operation, it is necessary to prevent the continuous operation of the pyrolysis furnace from being interrupted by lowered capacity of the heat exchanger, increased pressure drop, and blocking of the heat exchanger due to scaling.